In conventional mounting of semiconductor elements of a front-back conduction type, a back-side electrode of the semiconductor element is soldered to a substrate or the like, and a front-side electrode of the semiconductor element is wire-bonded. In recent years, however, mounting methods that involve direct soldering of a metal electrode of the front-side electrode of the semiconductor element have come to be frequently resorted to from the viewpoint of shortening manufacturing times and reducing material costs. The front-side electrode of the semiconductor element is generally formed of aluminum or of an aluminum alloy. Accordingly, a nickel film, gold film or the like must be formed on the front-side electrode of the semiconductor element, in order to perform soldering.
The nickel film needs to be several μm thick as reduction thereof occurs due to reactions with tin-based solder during soldering. However, only thicknesses of at most about 1.0 μm are ordinarily achieved in cases where a vacuum deposition method such as vapor deposition or sputtering is resorted to. Moreover, excessively increasing the thickness of the nickel film translates into higher production costs. Therefore, plating techniques have gained attention as film formation methods that allow the formation of thick films quickly and at low cost.
Plating techniques include electroless plating, which allows selectively forming a plating layer only at the surface of an electrode formed of aluminum or of an aluminum alloy (hereafter referred to as an “Al electrode”). Palladium catalyst methods and zincate methods are generally resorted to as electroless plating methods.
In a palladium catalyst method, an electroless plating layer is formed by causing palladium to precipitate, as a catalyst nucleus, on the surface of an Al electrode. Although a palladium method is advantageous in that the amount of etching of the Al electrode is small and the smoothness of the surface of the electroless plating layer is good, such a method incurs higher production costs, since palladium is a noble metal.
In a zincate method, an electroless plating layer is formed by substituting zinc for Al on the surface of an Al electrode, to elicit thereby precipitation of catalyst nuclei. Zincate solutions used in this method are inexpensive, and hence the method is widely used.
Actually, Patent Document 1 proposes the feature of selectively forming a nickel plating layer and a gold plating layer, by relying on a zincate method, on the surface of an Al electrode of a semiconductor element.
Patent Document 1: Japanese Patent Application Laid-Open No. 2005-51084